U.S. Pat. No. 3,853,510 describes an air filter comprising an inner filter element and an outer filter element, which can be inserted into each other to implement two-stage filtration. Both the inner filter element and the outer filter element are provided with a respective wedge-shaped filter medium that is tapered toward an end face, wherein the outer filter medium is larger than the inner filter element, so that an intermediate space between the inner and outer filter media is enclosed. The wedge angles of the inner and outer filter media are identical.
U.S. Pat. No. 3,204,391 also describes an air filter having an inner filter element and an outer filter element, which can be inserted into each other and each have a tapered design across the axial lengths thereof. The cone angle of the filter elements differs, so that an intermediate space having a variable cross-sectional geometry is formed between the inner and outer filter elements.